1. Field of the Invention
The present invention relates to communication systems, and, in particular, to aggregation of data frames in a packet-based network.
2. Description of the Related Art
Wireless local area networks (WLANs) include one or more fixed/non-fixed position stations (STAs, such as mobile terminals), including cell phones, notebook (laptop) computers, and hand-held computers, that are equipped with generally available WLAN PC cards. WLAN PC cards enable STAs to communicate among themselves when located within the same service area as well as through a network server when located in different service areas. The network server provides support for communication between STAs in different service areas that are supported by different access points (APs). An AP is a terminal or other device that provides connectivity to other networks or service areas, and may be either fixed or non-fixed. A basic service set (BSS) is formed between STAs and/or between an AP and one or more STAs. Such WLAN networks allow STAs to be moved within a particular service area without regard to physical connections among the mobile terminals within that service area.
An example of a WLAN network is a network that conforms to standards developed and proposed by the Institute of Electrical and Electronic Engineers (IEEE) 802.11 Committee (referred to herein as a network operating in accordance with one or more editions of the IEEE 802.11 standard). Typically, all messages transmitted among the mobile terminals of the same service area (i.e., those terminals associated with the same AP) in such WLAN networks are transmitted to the access point (AP) rather than being directly transmitted between the mobile terminals. Such centralized wireless communication provides significant advantages in terms of simplicity of the communication link as well as in power savings.
Most networks are organized as a series of layers (a layered-network architecture), each layer built upon its predecessor. The purpose of each layer is to offer services to the higher layers, shielding those layers from implementation details of lower layers. Between each pair of adjacent layers is an interface that defines those services. The lowest layers are the data link and physical layers. The function of the data link layer is to partition input data into data frames and transmit the frames over the physical layer sequentially. Each data frame includes a header that contains control and sequence information for the frames. The function of the lowest level, the physical layer, is to transfer bits over a communication medium.
FIG. 1 shows a prior art framing sequence for user data in accordance with an 802.11-compliant WLAN. As shown in FIG. 1, six protocol layers are shown as application layer 150, transmission control protocol (TCP) layer 151, Internet protocol (IP) layer 152, logical link control (LLC) layer 153 (a sub-layer in the data link layer), medium access control (MAC) layer 154 (also a sub-layer in the data link layer), and physical (PHY) layer 155 (the physical layer). User data 101 is provided to application layer 150, which generates application data 102 by appending application-layer header 110. Application data 102 is provided to TCP layer 151, which appends TCP header 111 to application data 102 to form TCP segment 103. IP layer 152 appends IP header 112 to TCP segment 103 to form IP frame 104. IP frame 104 might be a typical TCP/IP packet that is commonly employed in many data networking applications including some that are not necessarily 802.11-compliant.
LLC layer 153 provides a uniform interface between MAC layer 154 and higher layers, providing transparency of the type of WLAN used to transport the TCP/IP packet. LLC layer 153 appends this interface information as LLC header 113 to IP frame 104 to form LLC frame 105.
In 802.11-compliant WLANs, the physical device is a radio and the physical communication medium is free space. A MAC device and a PHY-layer signaling control device ensure two network stations are communicating with the correct frame format and protocol. The IEEE 802.11 standard for WLANs defines the communication protocol between two (or more) peer PHY devices as well as between the associated peer MAC devices. According to the 802.11 WLAN data communication protocol, each packet frame transferred between the MAC device and the PHY device has a PHY header, a MAC header, MAC data, and error checking fields. A typical format for the MAC-layer frame of 802.11-compliant WLAN systems appends MAC header 114 and frame check sequence (FCS) 115 to LLC frame 105 to form MAC frame 106. MAC header 114 includes frame control, duration identification (ID), source (i.e., MAC layer) and destination address, and data sequence control (number) fields. The data sequence control field provides sequence-numbering information that allows a receiver to reconstruct the data sequence order since the user data is a portion of a larger user data steam.
PHY layer 155 forms a physical-layer packet frame 107 by appending PHY header 118 to MAC frame 106. PHY header 118 includes preamble 116 and physical-layer convergence protocol (PLCP) header 117. PLCP header 117 identifies, for example, the data rate and length of PHY layer 155, and preamble 116 might be used by a receiving device to i) detect/synchronize to the incoming frame and ii) estimate the channel characteristics between the transmitter and receiver.
In typical communication between devices in accordance with the 802.11 standard, every MAC-layer frame is acknowledged with an ACK message (or ACK frame) that is sent a short interframe space (SIFS) period after the initial physical-layer frame is sent. In the 802.11e standard specification, some alternative acknowledgment methods are specified, such as “No-ACK” in which no acknowledgment message is sent at all. Other possible methods include variations of Block-ACK, where multiple data frames can be acknowledged in one Block-ACK message, which Block-ACK message is sent either immediately after a Block-ACK request (immediate Block-ACK) or after a separate contention period (delayed Block-ACK).
One factor that affects the maximum achievable throughput of 802.11 WLAN systems at a given layer is the length of the frames that carry the payload (e.g., user data 101). With relatively good channel quality, the throughput efficiency increases with increasing frame size. This increase in throughput efficiency is related to the fixed-size of the overhead (e.g., header, checksum) of the layer, since, as frame size increases, the ratio of overhead to data decreases. In some prior art 802.11-compliant WLANs, each transmitted PHY-layer packet frame contains exactly one MAC frame. MAC frames might be control or management frames, such as probe request or acknowledgment frames. Other MAC frames are data frames that contain exactly one packet of higher-layer user data. The MAC header and FCS field are included in the packet overhead.
The efficiency of the IEEE 802.11 protocol degrades when higher physical-layer data rates are used. This is caused by several sources of overhead, both on the PHY-layer and MAC-layer levels, such as preamble, inter-frame space (IFS) timing, and header information, and also acknowledgment packets. IEEE 802.11 proposes some methods to improve efficiency, such as burst packet transmission and Block-ACK messaging, but these methods do not enable efficient use of PHY-layer rates of 162 Mbit/s or higher.
At the PHY layer, overhead is introduced by the PHY-frame preamble and by the PLCP header, which are both of a constant size for each MAC data frame. Thus, the medium is more efficiently used if a larger amount of data is carried per packet. Also, because PHY-layer frame overhead is generally constant in time rather than in size (i.e., number of bytes), PHY layer frame overhead does not scale with higher data bit rates. Reducing the number of PHY frames for a given amount of transmitted user data can result in a significant improvement in efficiency. Because the PHY-layer overhead is included in every transmitted MAC frame, including those MAC frames not directly related to user data transfer (e.g., probe requests, RTS/CTS and Acknowledgment frames), reducing the number of separately transmitted MAC frames improves efficiency. Reducing the number of PHY frames also reduces the MAC contention overhead.